1. Field of the Invention
The present invention relates to an alignment device of a projection type exposure apparatus used in manufacture of a semiconductor device, and more particularly to a device for aligning a mask bearing an original pattern relative to a substrate such as a semiconductor wafer on which the original pattern is to be transferred.
2. Related Background Art
A projection type exposure apparatus (stepper) has been widely used as an apparatus for transferring a fine pattern such as a semiconductor device pattern onto a semiconductor wafer with a high resolution. In the prior art stepper, in order to align a reticle (or mask) to a shot area on the wafer, an alignment mark formed on a periphery of a circuit pattern of the reticle and an alignment mark formed in a periphery of the shot area on the wafer are simultaneously detected.
In the alignment system, the mark on the reticle and the mark on the wafer are detected with high precision, a relative positional deviation is detected, and the reticle or wafer is finely moved to compensate for the deviation. In the projection type exposure apparatus, in order to focus the pattern of the reticle onto the wafer with high resolution, a projection optical system is usually corrected for chromatic aberration to only an exposure illumination light (for example, g-ray having a wavelength of 436 nm or i-ray having a wavelength of 365 nm). This means that, in an alignment optical system for detecting the mark of the reticle and the mark of the wafer through the projection optical system, the mark illumination light is limited to the wavelength of the exposing light or one which is very close thereto.
The wafer in the exposure step has a resist layer formed thereon. In the alignment step, the mark on the wafer is detected through the resist layer. In order to allow formation of a pattern of higher resolution, the resist layer usually uses a multi-layer resist structure so that it exhibits a high absorption rate and a low transmission rate to the exposure light. In this case, the alignment illumination light is attenuated before it reaches the mark on the wafer, and reflected light (normal reflected light, scattered light or diffracted light) from the mark is also attenuated so that the mark on the wafer is not well detected by the alignment optical system.
It has been suggested to use light having a wavelength which exhibits a high transmission rate to the resist layer as the alignment illumination light In U.S.P. No. 4,492,459, a small lens for correcting chromatic aberration is arranged in an alignment light path between the reticle and the projection optical system so that the mark on the reticle and the mark on the wafer are conjugated to each other even under light having a different wavelength than the exposure light. In this system, if the correcting optical system is movable relative to the projection optical system, the alignment precision is extremely lowered because of an unstable factor in setting the correction optical system. Accordingly, they are fixed to each other. As a result, the mark on the reticle has to be sufficiently spaced from the area of the circuit pattern so that the correction optical system does not block a portion of the focusing light beam of the circuit pattern in the exposure step. Moreover, in a stepper which sequentially aligns the pattern of a reticle to each of a plurality of shot areas on the wafer by a step-and-repeat method, it is desirable that the alignment be attained for each shot area on the wafer.
The alignment precision will be improved if the detection resolution of the mark is improved, and a diffraction grating is known as a mark which permits the detection with highest precision. In this method, a positional error between the reticle and the wafer is detected based on a phase difference between photo-electric signals produced by moving diffracted light generated by a grating formed on the reticle as the mark and a grating formed on the wafer as the mark, along a direction of arrangements of the gratings. This method is disclosed in U.S.P. No. 4,251,160. In another method such as a proximity method, a positional error between the mask and the wafer is detected within a resolution of 1/several to 1/several tens of the pitch of the diffraction grating by optical information (sinusoidal intensity information) produced by interfering diffracted light generated by the grating on the mask with diffracted light generated by the grating on the wafer. In one experiment, the detection resolution of several nm has been reported in the alignment using the diffraction gratings.
The prior art projection type exposure apparatus having the correction optical system involves the following problem.
The shot areas on the wafer are usually partitioned by scribelines having a width of 50-100 .mu.m. If a mark associated with one shot area is to be formed on the scribe line spaced by 100 .mu.m from the circuit pattern area, the mark on the reticle is spaced from the circuit pattern area on the reticle by only 1000 .mu.m (1 mm) if a reduction factor of the projection optical system is 1/10. If the correction optical system is constructed by a small lens, a lens system having a diameter of only 2 mm (including a lens barrel) must be provided in order to prevent the focusing light path of the circuit pattern from being blocked. This is very impractical. In order to solve the above problem, U.S.P. No. 4,269,505 discloses a correction optical system which maintains a conjugate relationship between the mark on the reticle and the mark associated with the shot area on the wafer even if the mark on the reticle is largely spaced from the circuit pattern area.
However, the apparatus disclosed in U.S.P. No. 4,492,459 and 4,269,505 can not comply with change of the shot area on the wafer (the size of the circuit pattern on the reticle), and hence a construction to move the correction optical system (or a portion thereof) must be adopted, or a site alignment system in which relative positions of the reticle and the wafer are offset by a predetermined distance between the alignment step and the exposure step must be adopted.
The alignment system which uses diffraction gratings allows mark position detection with high resolution. However, in the technique disclosed in U.S.P. No. 4,251,160, the same problem as that raised by the provision of the correction optical system arises because an optical modulator is provided in the alignment light path between the reticle and the projection optical system. Although an alignment system which directly utilizes the periodic optical information uniquely defined by a periodic structure of the diffraction grating allows high precision detection, it is not a practical structure applicable to a projection type exposure apparatus, particularly a stepper.